1. Field of the Invention
The present invention relates to a hydrogen-absorption alloy electrode for use as a negative electrode in nickel-metal hydride rechargeable batteries.
2. Description of Related Art
Hydrogen-absorption alloys are capable of absorbing and releasing hydrogen at around room temperature and under around atmospheric pressure and are therefore used as a material for negative electrodes in nickel-metal hydride rechargeable batteries. Nickel-metal hydride rechargeable batteries are high in energy density as compared to nickel-cadmium rechargeable batteries and lead storage batteries and with their electrodes essentially free of harmful elements, have attracted much attention as clean batteries.
Hydrogen-absorption alloy electrodes make use of particles of a hydrogen-absorption alloy. Such particles are prepared by first roughly grinding an alloy ingot and then mechanically milling the ground alloy with an attritor into particles having a predetermined average size of, for example, about 50 μm.
One disadvantage of hydrogen-absorption alloys is that when exposed to air, the alloys readily form a layer of oxides, which prevents absorption and release of hydrogen by the alloy. To cope with this, several techniques have been developed through which a nickel metal layer is formed on the surface of the alloy particles to act as a hydrogen dissociation catalyst layer for enhancing the activity of the alloy. Among these techniques are (1) treatment of the hydrogen-absorption alloy in a high-temperature alkaline solution, (2) treatment in an acidic aqueous solution such as hydrochloric acid, and (3) electroless plating of Ni onto the alloy particles.
The mechanically milled hydrogen-absorption alloy particles, however, widely range in size from several microns to several tens of microns, and the size distribution of the particles tends to deviate significantly. This results in a significant deviation in self-discharging characteristics after charging/discharging cycles.
Also, when the particles of hydrogen-absorption alloy whose surfaces have been covered with a nickel metal layer are used in an electrode of batteries, the activity of the alloy and thus the initial battery characteristics remain low during initial cycles. To address this problem, the batteries are subjected to an initial activation process by repeating charging and discharging several times at a small current so that a predetermined capacity is ensured. However, this process requires a considerable amount of time and thus reduces productivity.